Problem: Let $X$ and $Y$ be sets: $X = \{12,2,3,15\}$ $Y = \{5,4,10,11\}$ What is the set $X \cup Y$ ?
Solution: Remember that $\cup$ refers to the union of sets. The union of two sets $X$ and $Y$ is the collection of elements which are in $X$ or in $Y$ or in both $X$ and $Y$ The members of a set must be unique, and the order doesn't matter. $X \cup Y = \{2,3,4,5,10,11,12,15\}$